1. Field of the Invention
This invention relates generally to dampers for regulating fluid flow. More particularly, the invention pertains to apparatus for dampening gas flows in heating, ventilation and air conditioning (HVAC) applications, including use as a smoke safety damper and/or a fire safety damper.
2. State of the Art
Variable flow dampers have been used for a long time to control air flow rates in heating, ventilating and air conditioning duct systems.
Depending upon the desired purpose, dampers may be quite simple or relatively sophisticated and complex.
A simple "volume damper" is merely a duct insert with one or more internal pivoting blades whose positions are set by a lockable hand lever. Such blades are sometimes referred to as vanes or louvers.
In a more sophisticated "motorized volume damper", the blade position is controlled between an open and a closed position by a shaft rotated by an actuating motor.
In a "smoke damper", the blades are activated when smoke is detected, either within the duct or at some remote location. Typically, the blade actuator motor is activated by a smoke detector to tightly close the blades for minimum leakage. Locking devices are provided to ensure that when in the closed position, the blades will not open without manual intervention, generally requiring access to the inside of the damper.
A "fire damper" is one which closes to prevent flames and high temperature gases from rapidly spreading within a building. Fire dampers are required by U.S. building codes to maintain the required fire resistance ratings of walls, partitions and floors wherever they are penetrated by an air duct. A fire damper must be operable to close even when electric power has been interrupted. Typically, a meltable fuse or thermostat releases the blades so that they automatically slam shut under gravitational force or by a spring at a predetermined temperature, typically about 165.degree. F. (74.degree. C.). In actual practice, the flame temperatures attained may destroy the elasticity of the biasing spring, making it useless for keeping the blades shut under the overpressures experienced.
Many fire dampers are built to be ON-OFF safety devices only, and have no function in general flow control.
It is the view of some in the industry that in most instances, current fire dampers merely act to provide a brief delay in the spread of the conflagration, but any delay time, however small, is of value in reducing injury or preventing loss of life. In any case, current fire dampers rarely survive a fire.
Some dampers are designed to shut under either a smoke detector signal or the presence of high temperature. These "smoke-and-fire dampers" combine the features of both damper types.
Volume dampers with single-hinged blades are shown in U.S. Pat. No. 594,727 of Cooper, U.S. Pat. No. 2,320,007 of Otto, U.S. Pat. No. 2,360,888 of Peple, Jr., U.S. Pat. No. 2,400,044 of Hermanson, U.S. Pat. No. 3,847,210 of Wells, U.S. Pat. No. 4,592,535 of Magill et al., and U.S. Pat. No. 4,472,999 of McCabe. None of these patents shows a damper configured as a smoke damper or fire damper, with the exception of the McCabe patent. In McCabe, a single spring biased blade is moved by a lever attached to a rotatable shaft. The lever/shaft connection is shown as a serpentine bimetallic element which when heated to a predetermined temperature, disconnects the lever from the shaft, permitting the blade to close. The damper may be used for maintaining an open position in the event of e.g. smoke detection; the spring position is altered to bias the damper blade to an open position. The damper cannot be used for opening the blade under one stimulus, i.e. smoke and closing it under another, i.e. fire, since the response depends upon the spring location. Springs installed for each action would cancel each other.
Flexible damper louvers comprising flexible tubular members expanded by internal pressure, movable rods or an engaging member are shown in U.S. Pat. No. 3,329,163 of Barker et al., U.S. Pat. No. 3,768,512 of Lahaye, and U.S. Pat. No. 5,123,435 of Blacklin et al. Practical use of the Barker et al. and Blacklin et al. inventions in a high temperature environment is difficult to envision, and the Lahaye apparatus requires a very complex control system. Furthermore, the flexible thin-skin metal or plastic vanes of Barker et al. and Lahaye will not be very resistant to fire and heat. In addition, repeated bending will lead to cracking and breakage.
U.S. Pat. No. 3,412,755 of Mason describes a pressure actuated valve for a duct wherein duct pressure closes the valve against a force exerted by springs on each side of the duct.
U.S. Pat. No. 3,847,210 of Wells discloses a gear system for simultaneously controlling three streams of gas.
U.S. Pat. No. 2,672,088 of Orr, U.S. Pat. No. 2,884,005 of Honerkamp et al., U.S. Pat. No. 3,958,605 of Nishizu et al., U.S. Pat. No. 4,457,336 of Allan et al. and U.S. Pat. No. 4,535,811 of Wood et al. appear to show dampers with hingedly interconnected blades of differing dimensions. No means for biasing the damper to an open or closed position is disclosed.
The Allan et al. and Wood et al. patents show systems where the blades fold into a framework with windows, and are actuated by a cammed drive.
In Nishizu et al., a four-member vane device with six hingepins and an internal biasing spring is used to maintain a constant airflow, regardless of upstream pressure. An external lever can be used to increase or decrease the spring tension.
In the Honerkamp et al. document, each vane device has four vane panels of unequal dimensions, and a side hinge pin of each vane device is connected to a transverse rod driven by a cam. The apparatus results in a requirement for high applied leverage forces to activate the damper.
The Orr reference describes a damper wherein four-member blade structures have side hinge pins connected to a common member which is moved transversely by a lever.
In each of the above references, the illustrated damper has various shortcomings which limit it use. Where two of the four members are substantially longer than the other members, the damper will not have a fail-safe closure, because increased upstream pressures may open the closure. This is illustrated in FIG. 1, showing a duct 200 wherein a four-member closure 204 of damper 202 has stationary hinge pin 206, drive hinge pin 208, and side pins 210, 212 as shown. When used as a fire damper, drive hinge pin 208 is driven by a spring or other biasing means 214 to close. If the damper 202 is mounted as shown with incoming gas stream 216, static gas pressure 218 against the blade members 220 may open the closed closure 204. If the damper 202 is mounted in the reverse order, i.e. for incoming gas stream 222, the static force 224 tending to open the closure 204 is much greater than the static force 226 tending to keep the closure closed. Thus, the damper 202 is not fail-safe in the event of, for example, loss of the required biasing spring force. Such might be expected in a fire.
The need for a damper which may be used as a true fail-safe smoke damper, fire damper, or combination smoke-fire damper in a variety of modes is evident.